A number of applications exist wherein it is desirable to limit access to machines to only those persons having proper authorization and further to limit the extent of such access. As one example, it is sometimes desirable to limit the use of copier machines to selected personnel. The copier machine is often provided with a substantially tamper proof counter for maintaining a cumulative count of the number of copies which have been produced to date. One standard technique for authorizing use of the copier machine is to provide selected personnel with a conventional key arranged to release a cooperating mechanical lock which, when opened, may be arranged to provide for the completion of an electrical circuit to provide power to the machine. Since keys of this type may be simply, readily and inexpensively reproduced, one standard prior art approach to avoid simple reproduction of keys is described in U.S. Pat. No. 3,436,530, issued Apr. 1, 1969 and assigned to the assignee of the present application, and which discloses a control unit forming a part of the copier machine and arranged to releaseably receive a portable plug-in unit containing an electromechanical counter and an electrical or electronic circuit element or elements which, when the portable unit is plugged into the control unit, serve to operate a relay which couples the machine to a local power source. The control unit is also preferably provided with an electromechanical counter.
The machine or device whose access is being controlled generates pulses representative of machine usage. In the copier machine example, each pulse preferably represents the reproduction of a single copy. Each pulse is simultaneously applied to the electromagnetic counters of the portable, plug-in unit and the control unit. Ideally, the counter of the control unit should contain a count equal to the grand total of the individual counts in each of the portable plug-in units used in conjunction with that copier machine.
The above mentioned accountability system, although reasonably satisfactory for cost allocation purposes, fails to provide any means for limiting the extent of access which any authorized holder of a portable, plug-in unit.
This limitation led to the development of the system described in U.S. Pat. No. 3,921,875, issued Nov. 25, 1975 to the assignee of the present application. The apparatus described in the last mentioned patent also comprises a control unit forming part of the machine and a portable, plug-in unit. The portable unit contains an electromechanical counter which may be preset to any desired count from 0 to a count of up to 99999, for example. The portable unit is also provided with an electrical circuit for operating a relay to couple the machine whose usage is being regulated to a local power source. Pulses generated by the machine and representative of machine usage are applied to the electromagnetic counter of the control unit and to the electromagnetic counter of the portable unit to respectively increment the control unit counter and decrement the portable unit counter. When the count in the portable unit counter has been decremented to a predetermined value (preferably 0) a switch is opened to deenergize the aforementioned relay and thereby decouple the machine from its source of power.
The count in the electromechanical counter of the plug-in unit may be reset by opening the plug-in unit through the use of a mechanical key. Alternatively, the count may be advanced through the use of a pulsing device which applies the number of pulses to the electromechanical counter equal to the desired count to be stored therein. This is a sluggish and tedious operation due to the electromechanical nature of the counter which is further subject to wearing over a period of continuous use. The portable plug-in units and control unit are also unnecessarily large and subject to wearing and breakdown due to the large number of mechanical moving parts utilized in these units.
In addition to the above disadvantages, the portable unit housing may be broken into rather easily and the electromagnetic counter mechanism may easily be tampered with to create a non-zero count condition, thereby destroying system security. The electromechanical nature of the device also lends itself readily to evaluation of the contents to further aid unauthorized personnel in their quest to defeat the security measures and thereby permit unauthorized use of the machine or device being protected against unauthorized use.